1. Field of the Invention
The present invention relates to a damper mechanism, particularly to a damper mechanism having two levels of torsion vibration dampening characteristics.
2. Description of the Related Art
A clutch disc assembly for use in, for example, an automotive vehicle is typically provided with a power input plate, a power output hub having a flange integral with its outer circumference, and coil springs for elastically coupling the power input plate and the flange in the circumferential direction.
Alternatively, a separate hub/intermediate plate type damper disc assembly have also been provided. In this type of damper disc assembly, the flange portion is made separate from hub defining an intermediate member. Typically, small coil springs elastically couple the hub portion and the separate flange. In this type of clutch disc assembly, a larger twist angular displacement can be produced, and further, two levels of the characteristics of torsion, low rigidity and high rigidity, can be obtained.
In order to produce hysteresis torque (torque produced due to frictional resistance and dampening vibration), a small hysteresis torque generating mechanism is disposed between the power input plate and the hub ,and a large hysteresis torque generating mechanism is disposed between the separated flange and the power input plate.
In such a separate hub/intermediate plate type damper disc assembly, the transmission of torsional vibration causes the regular relative rotation between the power input plate, the separated flange and the power output hub. At this time, the coil springs are compressed, thereby producing hysteresis torque in the small hysteresis torque generating mechanism and the large hysteresis torque generating mechanism.
As to the characteristics of torsion, during a small torsional displacement, the power input plate rotates together with the separate flange. This causes the power output hub to rotate relative to the power input plate and the separate flange. In this case, small coil springs are compressed, thereby producing small hysteresis torque in the small hysteresis torque generating mechanism. When the torsional displacement becomes larger, the separated flange rotates together with the power output hub. This causes the power input plate to rotate relative to the hub and the separate flange. In this case, large coil springs are compressed between the power input plate and the separated flange, thereby producing large hysteresis torque in the large hysteresis torque generating mechanism.
In the separate hub/intermediate plate type damper disc assembly described above, the power input plate is composed of a clutch plate and a retaining plate which are fixed to each other and disposed so as to clamp the separated flange and the power output hub. The small and large hysteresis torque generating mechanisms are composed of bushings (friction members) which are disposed between the plates, the power output hub and the separated flange and slidably move against the power output hub and the separated flange, and conical springs (biasing members) for biasing the bushings toward the power output hub and the separated flange.
The conventional small and large hysteresis generating mechanisms include first bushings having higher friction coefficient disposed on the retaining plate side of the separated flange, second bushings having lower friction coefficient disposed on the retaining plate side of the power output hub, and third bushings having higher friction coefficient disposed on the clutch plate side of the separated flange and the power output hub. In other words, the third bushings are configured to slidably move against both the separated flange and the power output hub. For this reason, small hysteresis torque is produced by sliding movement between the second bushing of lower friction coefficient and the face on the retaining plate side of the power output hub, and sliding movement between the third bushing of higher friction coefficient and the face on the clutch plate side of the power output hub. In this case, since the friction coefficient of the second bushing is different from that of the third bushing, the states of frictional sliding movement in the opposite sides of the power output hub are different and the value of the hysteresis torque is not stable. This degrades the performance of the damper disc assembly.
It may be possible to divide the third bushing into two components, that is, the bushings corresponding the first and second bushings and dispose them to the clutch plate side of the separated flange and the clutch plate side of the power out put hub, respectively. However, it is not desirable because it causes increase in number of components and operational inefficiencies.